WASHINGTON — Republican New Jersey Gov. Chris Christie boldly guaranteed yesterday that GOP presidential candidate Mitt Romney will win Wednesday’s first debate — while every other official from both campaigns tried to lower expectations.

The tough-talking governor declared that Romney, who is trailing in national and swing-state polls, will turn the race “upside down” after the showdown in Denver.

“I think what we need is a big and bold performance Wednesday night, and that’s what he’s going to give us,” Christie said on ABC’s “This Week.”